Thin autotensioner

ABSTRACT

An autotensioner comprises a torsion coil spring interposed between a base and a rocking arm, and a tubular bushing provided between an inside surface of the base and an outside surface of the rocking arm. The torsion coil spring pushes and biases the rocking arm toward the bushing, and brings the pushing direction substantially in coincidence with the axial load direction of the force, acting on a stepped bolt supporting the rocking arm, from a belt. When the belt tension becomes high, the arm axial center displaces slightly from the base axial center, an extremely large first damping force acts on the rocking arm. When the belt tension becomes low, the smaller second damping force acts on the rocking arm.

CROSS REFERENCES TO RELATED APPLICATION

This application is a divisional application of prior U.S. applicationSer. No. 09/961,365 filed on Sep. 25, 2001, which is herein expresslyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an autotensioner imparting a suitabletension to a timing belt of an automobile engine or a belt driving aplurality of auxiliary equipment.

2. Description of the Related Art

An autotensioner is employed in a belt drive mechanism for transmittingdrive power of an engine to a plurality of equipment by a single endlessbelt. It imparts a suitable tension to the belt and causes the vibrationof the belt caused by the fluctuation of the rotational speed and loadof the engine to attenuate. Due to this, the drive force of the engineis reliably transmitted to the equipment.

In general, an autotensioner is provided with a cup-shaped base fixed toan engine block etc., a rocking arm rotatably axially supported at thebase, and a pulley attached to the front end of the rocking arm andabutting against the belt. The rocking arm is rotationally biased in adirection tensing the belt by a torsion coil spring, which is housed inthe base and is provided substantially concentrically with the center ofrotation. Due to this, suitable tension is imparted to the belt.Further, a bushing or friction member is provided between the rockingarm and the base for frictionally sliding with at least one of the same.Due to the bushing, a frictional force forming a rotational resistanceoccurs when the rocking arm rotates relative with respect to the base,the rotation of the rocking arm is damped, and vibration of the belt isattenuated.

In recent years, along with the improved performance of engines, thefluctuation in the rotational speed of the engine and the load appliedto the belt has increased. The fluctuation of tension of the belt hasalso become larger. Therefore, when the damping force is small, thefluctuation in tension of the belt cannot be fully suppressed, and thisresults in vibration together with vibration of the belt. Therefore, toimprove the damping performance of the autotensioner, a higher dampingforce is required. In particular, it is preferable to make the dampingforce acting on the rocking arm when the belt is tense (first dampingforce) larger than the damping force acting on the rocking arm when thebelt is slack (second damping force).

To answer this demand, some damping mechanisms provided with thefriction member have been proposed. However, in the conventional dampingmechanisms, it is difficult to make the first damping force muchdifferent from the second damping force, and if the twisting torque ofthe torsion coil spring is raised to increase the damping force, notonly the first damping force, but also the second damping forceincreases and the timing of tensing of the belt becomes slower, that is,the problem arises of a fall in the ability to follow the belt.

Thus, in the conventional damping mechanisms, it was not possible tosimultaneously satisfy both the demands of improving the dampingperformance of the autotensioner and maintaining the ability to followthe belt well.

On the other hand, in recent years, along with the increasingly smallersize of engines, autotensioners have been required to be made smaller insize as well. If for example the base is made smaller in size, the spacewhere the torsion coil spring is housed becomes smaller so the torsioncoil spring which can be housed is also limited to a smaller one. On theother hand, along with the increasingly sophisticated performance ofengines in recent years, autotensioners have been required to have aconsiderable biasing force. For an autotensioner having a considerablebiasing force, a torsion coil spring having a considerable springconstant and a considerable spring biasing force is required. The springconstant and spring biasing force are determined by the coil length,wire diameter, etc. Therefore, for an autotensioner to have thenecessary biasing force, a space able to house a considerable spring isrequired. In an autotensioner, in which the size of the base is reduced,it is not possible to house a torsion coil spring having a sufficienttorque, and the biasing force to the belt may become insufficient.

Further, the friction member is required to be water resistant and toremain unchanged in frictional force even when sprayed with water, inaddition to being superior in heat resistance, abrasion resistance,strength, and dimensional stability. In the past, a synthetic resinsuperior in heat resistance, for example, a nylon resin, has been usedfor friction members. A friction member made of a nylon resin, however,increases in frictional force with the rocking arm when exposed to wateror salt water, so there was the problem that smooth rotation of therocking arm was obstructed and abnormal noise occurred from theautotensioner or belt.

SUMMARY OF THE INVENTION

Therefore, a first object of the present invention is to improve thedamping performance without reducing the following ability of theautotensioner.

According to the present invention, there is provided an autotensionercomprising a base, a rocking arm, a pulley, and a torsion coil spring.The base has a bottomed tubular shape. The rocking arm has a tubularpart rotatably supported at the inside of the base. The pulley isattached to one end of the rocking arm, and abuts against a belt. Thetorsion coil spring is housed in the base, and biases rotation of therocking arm in a direction tensing the belt with respect to the base.The torsion coil spring is attached eccentrically to the axial center ofthe base, and the rocking arm is supported so as to be able to bedisplaced relative to the base, whereby the first damping force actingon the rocking arm when the belt is tensed becomes relatively largerthan the second damping force acting on the rocking arm when the belt isslack.

Preferably, the rocking arm is attached movably in the radial directionto the base.

The autotensioner may be further provided with a friction memberinterposed between the outer circumferential surface of the tubular partof the rocking arm and the inner circumferential surface of the base,and provided across a range of at least 180 degrees around the axialcenter of the base, and part of the tubular part may be biased to bepushed against the friction member by the torsion coil spring. Due tothis, it is possible to generate a large frictional force.

The friction member may also be provided with a plurality of projectionsfor dispersing the load acting in a direction in which the torsion coilspring pushes and biases the arm tubular part. Due to this, it ispossible to prevent local abrasion and damage of the friction member.

The autotensioner may be further provided with a damping member separatefrom the friction member. Specifically, the damping member may engagewith the rocking arm movably in the radial direction and frictionallyslide with the base, so that a large damping force can be set.

The magnitude of the first damping force is preferably 1.5 to 3.5 timesthe magnitude of the second damping force.

A second object of the present invention is to obtain an autotensionerin which the thickness and size of a base is reduced while a sufficientbiasing force to the drive belt can be obtained.

According to the present invention, there is provided a thinautotensioner comprising a base, a rocking arm, and a torsion coilspring. The base has a cup having an inside diameter. The rocking arm isrotatably supported by the base. The torsion coil spring biases the armin a predetermined direction. The torsion coil spring has an outsidediameter larger than the inside diameter, and is twisted in a directionin which the outside diameter is compressed so as to be housed insidethe cup.

Preferably, the torsion coil spring is engaged at one end with a firstengagement part provided inside the base, and is engaged at the otherend with a second engagement part provided inside the arm. Theengagement part differs when the torsion coil spring engages with thefirst engagement part and the second engagement part and when therocking arm is attached to the base.

For example, an axial length of the torsion coil spring is shorter thanthe outside diameter.

The autotensioner may further be provided with at least one frictionmember interposed between the cup and the rocking arm and giving africtional resistance to the rocking of the rocking arm. The frictionmember may be composed of a tubular part, and a flange projecting from abottom of the tubular part to an inside direction of the cup and rockingarm, and exhibits an L-shape in cross-section.

Further, according to the present invention, there is provided a methodof assembly of a thin tensioner comprising a first step of twisting atorsion spring coil having an outside diameter larger than an insidediameter of a cup to make the outside diameter smaller than the insidediameter, and a second step of interposing the twisted torsion coilspring between the cup and rocking arm.

Furthermore, according to the present invention, there is provided amethod of assembly of a thin tensioner comprising a first step ofengaging one end of a torsion coil spring having an outside diameterlarger than an inside diameter of a cup with the cup, a second step ofengaging another end of the torsion coil spring with a rocking arm, athird step of rotating the rocking arm to twist the torsion coil springand make the outside diameter smaller than the inside diameter, a fourthstep of bringing the rocking arm into proximity with the cup to compressthe torsion coil spring and house it in the cup, and a fifth step ofrotatably fastening the rocking arm to the cup.

A third object of the present invention is to improve the frictionmember of an autotensioner and prevent the occurrence of abnormal noise.

According to the present invention, there is provided an autotensionercomprising a base, a rocking arm, and a friction member. The base has afirst tubular part having a bottomed tubular shape. The rocking arm hasa second tubular part, which is attached rotatably to an open side ofthe base and is separated by a certain distance from the first tubularpart in the radial direction. The friction member is provided betweenthe first tubular part and the second tubular part, and brakes therocking arm. The friction member is partially exposed, and is formedfrom a material mainly comprised of a polyphenylene sulfide resin.

A torsion coil spring may be provided at an inside of the second tubularpart of the rocking arm, to bias the rocking arm in a certain rotationaldirection and push the second tubular part and the friction membertoward the first tubular part. Due to this, it is possible to obtain africtional force changing in accordance with the tension of the belt bya simple configuration.

The autotensioner is preferably provided with a rocking shaft memberthat supports the rocking arm rotatably with respect to the base, andpasses through the bottom part of the base while forming a clearancewith respect to the base. Due to this, even if the friction member isabraded, it is possible to keep the rocking arm sliding with thefriction member at all times and a stable frictional force is obtained.

The friction member may be a tubular member partially cut away in thecircumferential direction, and can be manufactured relatively easily.Further, the friction member may have a plurality of grooves on itssurface and the surface frictionally slides with the rocking arm, thegrooves extending across the entire axis of the friction member. Byallowing abraded dust to escape through the grooves, damage to thefrictional sliding surface by the abraded dust is prevented.

The axial length of the first tubular part and the axial length of thesecond tubular part may be substantially equal, and the friction membermay be in close contact with the first tubular part and the secondtubular part across the entire axis of the friction member. Due to this,it is possible to increase the area of the receiving surface and receivea larger load.

Further, according to the present invention, there is a friction memberprovided in an autotensioner rotatably attaching a rocking arm to abase, characterized by being provided between the rocking arm and thebase, and being formed by a material mainly comprised of a polyphenylenesulfide resin. To set the material of the bushing to a frictionalcoefficient in accordance with the load to be applied, apolytetrafluoroethylene resin etc. is blended in the polyphenylenesulfide resin.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will be betterunderstood from the following description, with reference to theaccompanying drawings in which:

FIG. 1 is a view of a belt drive mechanism employing an autotensioner ofthe present invention;

FIG. 2 is a sectional view of a first embodiment of an autotensioneraccording to the present invention;

FIG. 3 is a plan view of the autotensioner shown in FIG. 2 seen from apulley;

FIG. 4 is a plan view of the autotensioner with a stationary beltwrapped around it;

FIG. 5 is an end view of an autotensioner along the line V-V of FIG. 4showing only a base, rocking arm, and stepped bolt;

FIG. 6 is a partially enlarged sectional view of FIG. 2 showing thestate of the bushing being sandwiched between the base and the rockingarm;

FIG. 7 is a perspective view of the bushing shown in FIG. 2;

FIG. 8 is a plan view of a torsion coil spring shown in FIG. 2;

FIG. 9 is a plan view of the state of the end of the torsion coil springattached to the base;

FIG. 10 is a plan view of the state of the end of the torsion coilspring attached to the arm;

FIG. 11 is a view of the dimensional differences before and afterassembly of the torsion coil spring;

FIGS. 12 a and 12 b are graphs of the output characteristics of theautotensioner shown in FIG. 1 in the case without the bushing and thecase with the bushing;

FIG. 13 is a view of a second embodiment of an autotensioner accordingto the present invention, showing part of the bushing and rocking armattached to the base;

FIG. 14 is a perspective view showing the bushing of FIG. 13 partiallycut away;

FIG. 15 is a view of a third embodiment of an autotensioner according tothe present invention, showing part of the bushing and rocking armattached to the base;

FIG. 16 is a perspective view showing the bushing of FIG. 15 partiallycut away;

FIG. 17 is a sectional view of a fourth embodiment of an autotensioneraccording to the present invention;

FIG. 18 is a perspective view showing the bushing of FIG. 17 partiallycut away;

FIG. 19 is a view of the state of measurement of the output load of anautotensioner;

FIG. 20 is a view of the result of measurement of an output load of anautotensioner; and

FIG. 21 is a graph of the results of a water spraying test.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described below with reference to theembodiments shown in the drawings.

FIG. 1 is a view of a belt drive mechanism employing an autotensioneraccording to a first embodiment of the present invention. The belt drivemechanism has a single endless belt 10. This belt 10 is wrapped around adrive pulley 12 attached to an output shaft of an engine (not shown),and driven pulleys attached to a plurality of apparatuses, for example,an air-conditioner pulley 14, a power steering system pulley 16, and analternator pulley 18. When the drive pulley 12 rotates, the belt 10rotates in the clockwise direction of the drawing and the rotationaldrive force is transmitted to the pulleys 14, 16, and 18 of theequipment.

The autotensioner 20 is arranged in proximity to the drive pulley 12,more particularly in the belt rotational direction after the drivepulley 12 where the belt 10 most easily becomes slack. The pulley 22 ofthe autotensioner 20 abuts against the back surface of the belt 10.Namely, the pulley 22 contacts the outer circumference side of the belt,while the pulley 22 rotates about the rotational axis thereof. Therocking arm 24 biases the pulley 22 to rotate in the arrow B directionin which the belt 10 is tensed, so that a suitable tension is applied tothe belt 10 at all times.

When the belt 10 vibrates due to fluctuation of the engine rotationalspeed or load, the vibration is transmitted to the rocking arm 24through the pulley 22, the rocking arm 24 rocks about the axial centerL4 of rocking, and the pulley 22 is moved relatively between the firstposition shown by the solid line and the second position shown by thebroken line. When the rocking arm 24 is rocking, the rocking arm 24 andthe bushing (or friction member) 26 frictionally slide and thefrictional force generated acts as a damping force braking the rockingarm 24. Thus, the relative movement of the pulley 22 is suppressed, andthe vibration of the belt 10 is attenuated.

If the tension of the belt 10 rapidly increases and the pulley 22 ispushed toward the second position, the rocking arm 24 rotates relativelyin the clockwise direction (arrow A direction). At this time, arelatively large first damping force acts on the rocking arm 24, so thatthe pulley 22 moves slowly, and effectively suppresses the vibration ofthe belt 10. Conversely, if the belt 10 becomes slack and the pulley 22moves toward the first position following the belt 10, the rocking arm24 rotates relatively in the counterclockwise direction (arrow Bdirection) about the axial center L4 of rocking. At this time, arelatively small second damping force acts on the rocking arm 24, sothat the pulley 22 moves quickly toward the belt 10 to tense the belt10.

FIG. 2 is a sectional view of the autotensioner 20, while FIG. 3 is aplan view of the autotensioner 20 seen from the pulley 22 side. In FIG.3, part of the rocking arm 24 is shown cut away, while the pulley 22 isshown by a one-dot chain line.

The autotensioner 20 is provided with a base 30 formed from an aluminumalloy or other metal material integrally in a bottomed tubular shape.The base bottom 32 is fixed to the engine block (not shown). The basetubular part 34 extends perpendicularly from the outer circumferentialedge of the base bottom 32. The inside is formed in a stepped shape.Namely, the inside is formed with two tubular surfaces based on the baseaxial center L1 and having different diameters, i.e., the open sideinner circumferential surface 34 a and the bottom side innercircumferential 34 b, and the ring-shaped seat 34 c linking these twoinner circumferential surfaces 34 a and 34 b. The diameter D1 of theopen side inner circumferential surface 34 a is larger than the diameterD2 of the bottom side inner circumferential surface 34 b, while thering-shaped seat 34 c is a plane having a certain width D3 andperpendicular to the base axial center L1.

A circular base shaft hole part 38 is formed at the center of the basebottom 32. A stepped bolt 40 is inserted from the bottom in FIG. 2 atthe inside of the base shaft hole part 38. The rocking arm 24 isattached rockingly to the base 30 by the stepped bolt 40. In the statewithout the belt 10 wound around the pulley, the axial center of thestepped bolt 40 and the axial center L4 of rocking of the rocking arm 24substantially coincide with the base axial center L1.

The rocking arm 24 is formed integrally from an aluminum alloy or othermetal material, and exhibits a bottomed tubular shape open toward thebase bottom 32. The arm bottom 242 is arranged at the inside of theopening of the base 30. A tubular rocking shaft 244 extending toward thebase bottom 32 is arranged at the center. The rocking shaft 244 isopened at the two ends, and a female thread 244 a is formed at the innercircumferential surface thereof. By the screwing of the female thread244 a with the male thread 42 formed at the front end of the steppedbolt 40, the stepped bolt 40 and the rocking arm 24 are integrallyfastened.

The front end of the rocking shaft 244 enters the base shaft hole part38, and abuts against the cylindrical part 46 of the stepped bolt 40.Part of the cylindrical part 46 is arranged at the inside of the baseshaft hole part 38, and has the same outside diameter as the rockingshaft 244. A clearance is provided between the rocking shaft 244 andcylindrical part 46 and the base shaft hole part 38, so that the rockingarm 24 can rotate relatively without interference with the base 30.

The head 44 of the stepped bolt 40 is a disk having an outside diameterlarger than the inside diameter of the base shaft hole part 38, and isengaged with the base bottom 32. Explaining this in more detail, a boltreceiving part 33 defined by a cylindrical hole larger than the head 44is formed in the base bottom 32. The bolt receiving part 33 communicateswith the base shaft hole part 38, and opens downward in FIG. 2. The head44 is housed in the bolt receiving part 33 so as not to project out fromthe base bottom 32. The rocking arm 24 is biased in a direction (upwardin FIG. 2), separating the arm 24 from the base 30, by the torsion coilspring 60 compressed in the base axial center L1 direction. By theengagement of the head 44 with the bolt receiving part 33 through athrust bearing 50, relative movement of the rocking arm 24 along thebase axial center L1 direction is limited.

The thrust bearing 50 is a ring-shaped member provided between the head44 of the stepped bolt 40 and the bolt receiving part 33, and makes thehead 44 and the bolt receiving part 33 rotate smoothly relative to eachother. The thrust bearing 50 is formed from for example a syntheticresin material having a self-lubrication property.

The inside diameter D12 of the base shaft hole part 38 is formed to beslightly larger than the outside diameter D11 of the rocking shaft 244and the bolt cylindrical part 46. Further, the inside diameter D14 ofthe bolt receiving part 33 is formed to be slightly larger than theoutside diameter D13 of the bolt head 44. Due to this, the rocking arm24 and the stepped bolt 40 can smoothly rotate relatively withoutinterference with the base 30. Further, slight displacement of the axialcenter L4 of rocking with respect to the base axial center L1 isallowed.

The rocking arm 24 has a pulley bearing 248 projecting out to theopposite side of the base 30 from the arm bottom 242. The pulley 22 isrotatably attached to the outside of the radial direction of the pulleybearing 248 through a ball bearing 70. The axial center L2 of rotationof the pulley 22 is parallel to the axial center L4 of rocking. The ballbearing 70 is fastened to the pulley bearing 248 by the mounting bolt 74screwed inside the pulley bearing 248 and the washer 72 interposedbetween the head of the mounting bolt 74 and the top end of the ballbearing 70.

An arm tubular part 246 extending toward the base bottom 32 isintegrally provided at the outer circumferential edge of the arm bottom242. The arm outer circumferential surface 246 a is parallel to the openinner circumferential surface 34 a inside the base tubular part 34, andfaces it across a predetermined distance. A tubular bushing 26 incontact with the two surfaces is provided across the entire axis betweenthe arm outer circumferential surface 246 a and the open side innercircumferential surface 34 a.

The arm inner circumferential surface 246 b has a diameter equal to thediameter D2 of the base side inner circumferential surface 34 b of thebase 30, and is positioned on the same tubular surface as the bottomside inner circumferential surface 34 b in the state with the rockingarm 24 attached to the base 30. A ring-shaped chamber 100 of an outsidediameter D2 is formed by the base bottom 32, the arm bottom 242, thebottom side inner circumferential surface 34 b of the base 30, the arminner circumference 246 b, the base shaft hole part 38, and the rockingshaft 244. The torsion coil spring 60 is housed in the ring-shapedchamber 100. An axial length of the torsion coil spring 60 is shorterthan an outside diameter of the torsion coil spring 60.

The torsion coil spring 60 is a coil spring wound near the arm innercircumferential surface 246 b. One end 62 of the torsion spring 60 isengaged with the base bottom 32, while the other end 64 is engaged withthe arm bottom 242. The torsion coil spring 60 is interposed in thestate twisted by a predetermined angle in the clockwise direction ofFIG. 1 where the coil diameter is compressed, and compressed in the baseaxial center L1 direction. Due to the twisting torque of the torsioncoil spring 60 for elastic return in a direction increasing the coildiameter, the rocking arm 24 is biased to the counterclockwise directionof FIG. 1 about the base axial center L1, so that a predeterminedtension is imparted to the belt 10 wound around the pulley 22.

Since the torsion coil spring 60 is a coil spring, the reaction force ofthe twisting torque does not uniformly act about the base axial centerL1, and a part of the arm tubular part 246 is biased and pushed toward aportion of the bushing 26 and the base tubular part 34 at the outside inthe radial direction due to the torsion coil spring 60. In the statewith the belt 10 wrapped around the pulley 22 (see FIG. 3), a frictionalforce is generated between the arm tubular part 246 and the bushing 26by the combined force of the force of the belt 10 pushing the pulley 22and the biasing force of the torsion coil spring 60.

For example, if the tension of the belt 10 falls, the force receivedfrom the belt 10 falls, so the frictional force becomes smaller, theability of the pulley 22 to follow the belt 10 is raised, and a fall intension of the belt 10 is prevented. Conversely, if the tension of thebelt 10 increases, the force received from the belt 10 also increases,the frictional force becomes greater, and the rocking of the rocking arm24 is attenuated. Thus, the magnitude of the frictional force changesrelatively at the autotensioner 20.

With reference to FIG. 4 and FIG. 5, an operation of the autotensioneris described below. FIG. 4 is a plan view of an autotensioner 20 when astationary belt 10 is wrapped around it, while FIG. 5 is an end viewalong the line V-V passing through the base axial center L1. Note that,in FIG. 5, to avoid complicating the drawing, only the rocking arm 24,base 30, and stepped bolt 40 are shown. The rest of the configuration isomitted.

When the belt 10 pushes the pulley 22, a load is applied to the steppedbolt 40 and the rocking arm 24 in an axial load direction Y parallel tothe line P equally splitting in two the belt winding angle γ. At thistime, as shown in FIG. 5, a moment trying to tilt the axial center L4 ofrocking from the base axial center L1 about the base bottom 32 side actson the rocking arm 24. As explained above, a clearance is providedbetween the rocking shaft 244 and bolt cylindrical part 46 and the baseshaft hole part 38 and between the bolt head 44 and the bolt receivingpart 33. Therefore, due to the pushing force of the belt 10, the axialcenter L4 of rocking moves relatively slightly to the axial loaddirection Y in the plane perpendicular to the base axial center L1 (FIG.4), and tilts slightly to the axial load direction Y side, strictlyspeaking, rotates (FIG. 5).

In fact, since the bushing 26 and thrust bearing 50 are interposedbetween the base 30 and the rocking arm 24, as shown in FIG. 5, the tiltis not large enough to be able to be visually discerned. The tilt angleθ is an extremely small value.

The torsion coil spring 60 biases the arm tubular part 246 and thebushing 26 in the pushing direction Z, and pushes against the basetubular part 34. The pushing direction Z is substantially the samedirection as the axial load direction Y. Due to this, the force by whichthe rocking arm 24 pushes the bushing 26 becomes the total of thebiasing force of the torsion coil spring 60 in the pushing direction Zand the pushing force of the belt 10 in the axial load direction Y. Thebushing 26 is strongly sandwiched by the arm tubular part 246 and basetubular part 34, and in FIG. 4, a local force concentrates on thepushing portion 26 w shown by hatching. The pushing direction Z does notstrictly have to coincide with the axial load direction Z, but the rangeis preferably in the range of ±20 degrees around the base axial centerL1 with respect to the axial load direction Y (the counterclockwisedirection being made “forward”).

If the tension of the belt 10 is increased in the state with the belt 10wound around the pulley, the pushing force of the belt 10 in the axialload direction Y increases, and the rocking arm 24 rotates relatively inthe clockwise direction of FIG. 4. Therefore, the torsion coil spring 60elastically deforms in the direction compressing the coil diameter, sothat the reaction force of the twisting torque increases. Along withthis, the biasing force of the torsion coil spring 60 in the pushingdirection Z also increases. Therefore, the force substantiallycoinciding with the sum of the two forces, i.e., the force of therocking arm 24 pushing the pushing position 26 w of the bushing 26outside in the radial direction, becomes relatively larger.

The frictional force occurring between the rocking arm 24 and thebushing 26 is proportional to the vertical load acting on the contactsurfaces which are the rocking arm outer circumferential surface 246 aand the bushing inner circumferential surface 26 a, or, is proportionalto the force pushing outside in the radial direction. As explainedabove, since this vertical load becomes relatively large when the belttension increases, a large frictional force occurs. Further, since thebushing 26 is strongly pushed to the open side inner circumferentialsurface 34 a of the base 30, a large frictional force is generatedbetween the bushing outer circumferential surface 26 b and the open sideinner circumferential surface 34 a. Therefore, when the rocking arm 24rotates relatively in the clockwise direction of FIG. 4, a relativelylarge frictional resistance acts on the rocking arm 24 as the firstdamping force. Due to this, the rocking arm 24 is strongly braked, thepulley 10 slowly follows the belt 10, and the vibration of the belt 10is attenuated.

Further, as shown in the partially enlarged view of FIG. 6, if the axialcenter L4 of rocking is tilted by the angle θ, the rocking arm tubularpart 246 is not placed parallel to the base tubular part 34, but istilted to the axial load direction Y by the angle θ (in FIG. 6, rotatestoward the bottom left). In other words, near the pushing position 26 w,the distance between the open side inner circumferential surface 34 aand the arm outer circumferential surface 246 a becomes graduallysmaller toward the opening of the base 30. Thus, the pushing position 26w is more strongly sandwiched the further toward the opening, much likea wedge, by the open side inner circumferential surface 34 a and the armouter circumferential surface 246 a. Therefore, a so-called wedge-effectlike action is added to the frictional resistance caused by the pushingaction, and therefore a large damping force can be stably exerted. Notethat, in FIG. 6, for the explanation, the degree of tilt is shownexaggeratedly. In fact, the tilt is not as great as illustrated.

On the other hand, when the tension of the belt 10 is reduced, the forcereceived from the belt 10 is reduced, the rocking arm 24 rotatesrelatively in the counterclockwise direction of FIG. 4 by the twistingtorque of the torsion coil spring 60, and the torque coil spring 60elastically deforms in the direction increasing the coil diameter, sothe twisting torque is reduced. Due to this, the rocking arm 24 movesrelatively so that the axial center L4 of rocking coincides with thebase axial center L1, and the force by which the rocking arm 24 pushesthe bushing 26 in the pushing direction Z becomes extremely small.Further, since the degree of tilt of the rocking arm 24 becomes smalland the open side inner circumferential surface 34 a and arm outercircumferential surface 246 a are placed substantially parallel, theforce sandwiching the pushing position 26 w also becomes small and theforce applied to the bushing 26 is released. Therefore, the phenomenonsuch as the above wedge effect is eliminated, and the frictional forceis reduced by an extreme amount. In this way, when the rocking arm 24rotates relatively in the counterclockwise direction of FIG. 4, thesecond damping force acting on the rocking arm 24 is suppressed to a lowlevel, and the rocking arm 24 is not braked that much, so the ability ofthe pulley 22 to follow the belt 10 becomes higher, and a predeterminedtension is quickly imparted to the belt 10.

Thus, according to the autotensioner 20 of the embodiment, by displacingthe rocking arm 24 in accordance with the direction of rotation, it ispossible to relatively change the damping force, and possible toeffectively dampen vibration without reducing the tension of the belt10.

In a conventional device, the clearance between the base (30) and therocking arm (24) or stepped bolt (40) rotating relative to the base (30)was filled completely by a synthetic resin bushing. Tilt of the rockingarm (24) or relative displacement in the radial direction were notallowed. Therefore, the force pushing the bushing (26) outside in theradial direction was substantially constant without regard as to therotational direction of the rocking arm 24, and it was difficult to makethe first damping force and second damping force much different. In theautotensioner 20 of the embodiment, however, by providing a clearanceallowing displacement of the axial center L4 of rocking between therocking arm 24 and stepped bolt 40 and the base 30, it is possible togreatly change the force by which the rocking arm 24 pushes the bushing26, and it is possible to increase the difference between the first andsecond damping forces. Such an autotensioner 20 does not require theaddition of any new parts or productions steps compared with aconventional device. Conversely, since it does not require precision inprocessing the rocking shaft 244 and the shaft hole part 38, theproduction becomes easy.

Further, the amount of abrasion of the pushing portion 26 w of thebushing 26 is larger than that of other portions, but since the rockingarm 24 is displaceable, even if the thickness of the pushing portion 26w is reduced, the rocking arm 24 and stepped bolt 40 can displaceslightly with respect to the base 30 in the direction of the reductionof thickness by the biasing force of the torsion coil spring 60(matching with pushing direction Z in FIG. 4), and it is possible tokeep the bushing 26 and the rocking arm 24 in close contact at alltimes. Therefore, a stable frictional resistance is obtained.

FIG. 7 is a perspective view of the bushing 26. The bushing 26 is atubular member integrally formed by injection molding from a syntheticresin material mainly comprised of for example polyphenylene sulfide.Polyphenylene sulfide is a synthetic resin having a highly crystallinepolymer structure, and is superior in heat resistance, abrasionresistance, strength, and dimensional stability and extremely low inwater absorption. Therefore, by using polyphenylene sulfide for thebushing 26, the bushing 26 can prevent an increase in the frictionalforce even when sprayed with water or salt water. Note that, thematerial may have added to it, in addition to the polyphenylene sulfideresin, materials for adjusting the frictional coefficient in accordancewith the load to be received, for example, a polytetrafluoroethyleneresin or molybdenum for imparting a self-lubrication property, a heatstabilizer, an antioxidant, or a UV degradation preventer. Further, asthe main ingredient of the material, it is also possible to use theconventionally used polyether sulfone.

The bushing 26 is partially cut away in the circumferential direction.The cut away part 26 c allows expansion or contraction due to changes intemperature. Further, a flange 262 of a predetermined width extendinginside in the radial direction is formed over substantially the entirecircumference of the end of the bushing 26 at the base 30 side. Theflange 262 is sandwiched between the front end surface of the armtubular part 246 and the ring-shaped seat 34 c of the base 30, toprevent abrasion due to contact of the two and prevent detachment of thebushing 26 from the base 30. The bushing 26 is slightly larger inoutside diameter than the inside diameter D1 of the base tubular part 34in natural length, and is interposed inside the base tubular part 34 inthe state compressed in the radial direction. Therefore, the bushing 26closely contacts the open side inner circumferential surface 34 a of thebase 30 due to the force for expansion in the radial direction. Thebushing 26 has an axial length substantially the same as the axiallength H1 of the tubular part 246 of the rocking arm 24 and the basetubular part 34, and brings the base tubular part 34 and arm tubularpart 246 into close contact over the entire axis.

A plurality of grooves 268 extending over the entire axis are formed atthe inner circumferential surface 26 a of the bushing 26. The grooves268 function to collect the abraded dust produced when the bushing 26rubs against the rocking arm 24 and releases it outside. Due to this,abrasion of the inner circumference 26 a due to the abraded dust isprevented. Note that the sectional shape of the grooves 268 issemicircular in the embodiment, but is not particularly limited to asemicircular and may also be a longitudinal sectional shape, triangularsectional shape, etc. Further, the depth of the grooves is notparticularly limited, but if too deep, flexing occurs, while if tooshallow, abraded dust collects in the grooves 268, so it is necessarythat the grooves have a suitable depth. In the same way for the groovewidth, if too large, the necessary frictional force is not obtained,while if too narrow, the abraded dust collects in the grooves 268, sothe grooves have to be set to a suitable width.

The cut away part 26 c of the bushing 26 is arranged at the oppositeside of the pushing position 26 w with the largest abrasion (see FIG.3), i.e., the top right in FIG. 3. Due to this, expansion in the radialdirection due to abrasion is easy, so that a stable frictional force canbe obtained. Note that, in the embodiment, the bushing 26 is a partiallycut away tubular shape, and it is sufficient to set it in a range forpushing by the rocking arm 24. Specifically, it should be set over arange of ±90 degrees around the base axial center L1 from the axial loaddirection Y.

The autotensioner 20 of the embodiment exhibits a thin shape with adiameter of the base 30 which is relatively large with respect to itsaxial length, and a bushing (or friction member) 26 is provided at theoutside of the torsion coil spring 60. Therefore, the diameter of thebushing 26 is relatively large, and the area of the frictional surfacewith the rocking arm 24 can be set large. Further, since the bushing 26is in close contact with the arm tubular part 246 over the entire axis,the area of the frictional surface can be made large. Therefore, even ifthe rotational angle of the rocking arm 24 is small, a relatively largefrictional force can be obtained. Further, since the working position ofthe frictional force can be set further from the axial center L4 ofrocking (or the base axial center L1), the rocking arm 24 can beeffectively braked.

The rocking arm 24 frictionally sliding with the bushing 26 is formed byan aluminum alloy. When forming the bushing 26 from a nylon resin etc.as in a conventional device, the frictional force with respect to therocking arm 24 increases when the bushing 26 is exposed to water or saltwater, so there is the problem that smooth rotation of the rocking arm24 is obstructed, and abnormal noise such as belt squeaking occurs. Inthe embodiment, the main ingredient of the bushing 26 is changed fromnylon resin to polyphenylene sulfide resin. Due to this, it is possibleto prevent an increase in the frictional force even when the bushing 26is exposed to water or salt water. This is considered to be because apolyphenylene sulfide resin has a highly crystalline polymer structureand a low hydrophilicity.

The pulley side end 264 of the bushing 26 is exposed to the outside, soallows water or salt water to enter the interface with the base tubularpart 34 and the arm tubular part 246, and the outer surface is easilyexposed to water or salt water. Nevertheless, since the bushing 26 isformed by a material mainly comprised of poly-phenylene sulfide resin,the frictional force does not increase even if exposed to water, smoothrocking of the rocking arm 24 is not obstructed, and the stick-slipphenomenon or belt squeaking is prevented.

FIG. 8 is a plan view of the torsion coil spring 60 seen from therocking arm 24 side. The torsion coil spring 60 is provided with aspiral part 66 having a diameter D4 in the no-load state. The number ofturns of the spiral part 66 is about 2.2. One end 62 engaged with thebase 30 extends straight from the spiral part 66, and is at the spiralaxial center L3 side from the spiral part 66 and perpendicular to thespiral axial center L3. The other end 64 engaged with the rocking arm 24has the same structure as the one end 62. In the plane perpendicular tothe spiral axial center L3, the angle α formed by the line K1 extendingto the bent part 63 side through the center of the one end 62 and theline K2 extending to the bent part 65 side through the center of theother end 64 is about 60 degrees, but a range of 50 to 80 degrees ispreferable.

FIG. 9 is a plan view of the state of attachment of the torsion coilspring 60 to the base 30 seen from the rocking arm 24 side. Note thatthe torsion coil spring 60 is shown by hatching, and that only one turnat the base 30 side is shown. Two engagement projections 322 and 324gripping the one end 62 are formed at the base bottom 32. The firstengagement projection 322 exhibits a crescent shape projecting out frominside the base tubular part 34. The side surface 322 a, which is a flatplane, supports the one end 62 from the outside in the radial direction.The second engagement projection 324 exhibits a ring shape providedacross the entire circumference of the base shaft hole part 38, andabuts against the one end 62 from the inside. The outer circumferentialsurface of the second engagement projection 324 partially projects outin an arc at the outside in the radial direction. The projecting curvedsurface 324 b abuts against the bent part 63 of the torsion coil spring60. Thus, the one end 62 is engaged with the base bottom 32, and in theengaged state, the one end 62 is substantially parallel to the axialload direction Y.

The outside diameter D4 (see FIG. 8) of the spiral part 66 is largerthan the diameter D2 of the ring-shaped chamber 100 in which the part isto be housed. In the state where one end 62 is engaged with the base 30and the end 62 is made a free end so that no load is applied to the end62, the spiral axial center L3 does not coincide with the base axialcenter L1, and is offset to a position deviated left downward in thedrawing. The eccentric direction of the spiral axial center L3 withrespect to the base axial center L1 is substantially the same directionas the axial load direction Y.

FIG. 10 is a plan view of the state of attachment of the torsion coilspring 60 to the rocking arm 24 seen from the base 30 side. In thedrawing, the torsion coil spring 60 is shown by hatching, and only oneturn of the rocking arm 24 side is shown. The arm bottom 242 is providedwith a crescent-shaped third engagement projection 243 projecting out tothe inside from the arm tubular wall 246 and a crescent-shaped fourthengagement projection 245 provided at a rear side of the pulley bearing248. The third engagement projection 243 supports the other end 64 ofthe torsion coil spring 60 from the outside of the radial direction,while the fourth engagement projection 245 supports the bent part 65formed by the connecting part of the other end 64 and the spiral part 66from the inside of the radial direction. Thus, the other end 64 of thetorsion coil spring 60 is engaged with the rocking arm 24.

When assembling the autotensioner 20, first, one end 62 of the torsioncoil spring 60 is engaged with the base 30 in which the bushing 26 isfit (first step). With the other end 64 in the free end state, thespiral part 66 cannot be housed in the base 30, so next the rocking arm24 is placed over the torsion coil spring 60, and the other end 64 isengaged with the third and fourth engagement projections 243 and 245(second step). By rotating the rocking arm 24, the torsion coil spring60 is twisted in a direction in which the diameter of the spiral part 66is reduced (clockwise direction of FIG. 9) (third step). Due to this,the outside diameter of the spiral part 66 becomes smaller than thediameter D2, so that the part can be housed in the ring-shaped chamber100. After that, the rocking arm 24 is pushed to the base 30 side tocompress the torsion coil spring 60 in the base axial center L1direction (fourth step), then the stepped bolt 40 is screwed into therocking arm 24 in the compressed state, and the rocking arm 24 is fixedrotatably to the base 30 (fifth step). Further, the pulley 22, ballbearing 70, washer 72, and mounting bolt 74 are assembled (sixth step).Due to the above first to sixth steps, an autotensioner 20 in the stateshown in FIG. 2 is obtained.

In general, the size of the base 30 housing the torsion coil spring 60is determined by the size of the mounting space assigned to theautotensioner 20. The size of the torsion coil spring 60, or the limitsof the outside diameter and axial direction length, is determined byitself in accordance with the size of the base 30. In recent years,along with the increasingly small size of engines, the mounting space ofthe autotensioner 20 has also become narrower, and the size of the base30 has become smaller. On the other hand, the load on the belt 10 tendsto increase along with the improvement in performance of the engine.Therefore, an increase in the output load to be imparted to the belt 10,or an increase in the spring torque, is sought for the autotensioner 20as well. However, the spring torque is proportional to the coilthickness of the torsion coil spring 60, so if the base 30 is madesmaller, the output load falls, while if the output load is set high,there is the problem that the base 30 has to be made larger.

In a conventional device, a torsion coil spring was prevented fromcontacting the base or rocking arm by housing in the base the torsioncoil spring provided with an outside diameter smaller than the insidediameter of the base or rocking arm and twisting it by a predeterminedangle to bias the rocking arm. By twisting, the outside diameter of thetorsion coil spring becomes further smaller, so the clearance betweenthe torsion coil spring and the base becomes larger, which causes thehousing space (corresponding to the ring-shaped chamber 100) to be usedineffectively. Therefore, the inventors took note of the point thattwisting results in the outside diameter of the torsion coil springbecoming smaller, and discovered the method of twisting and then housingthe torsion coil spring 60 of the outside diameter D4 a certain degreelarger than the diameter D2 of the ring-shaped chamber 100 so as to makeeffective use of the ring-shaped chamber 100. Due to this, the effectwas obtained that even if the volume of the ring-shaped chamber 100 isthe same as in the conventional device, it is possible to use a torsioncoil spring 60 longer than in the conventional device, and that it ispossible to increase the output load without enlarging the base 30 orthe rocking arm 24. Further, the number of assembly steps is nodifferent from the conventional device.

The configuration of the torsion coil spring 60 is effective in the caseof application to a thin autotensioner 20 with an outside diameterrelatively larger than the axial length such as in the embodiment. Thereason is that when enlarging the outside diameter without changing theaxial length of the torsion coil spring 60, even if the amount ofincrease of the outside diameter is the same, the amount of increase ofthe coil length becomes larger the larger the diameter of the torsioncoil spring 60. Further, in the case of a thin model autotensioner,since the axial length of the base 30 is short, the work of tilting andhousing the torsion coil spring 60 and engaging its one end with thebase 30 becomes extremely easy. Further, since the amount of compressionof the outside diameter is large even at the same twisting anglecompared with a small diameter torsion coil spring, it is possible tosufficiently compress the diameter by just twisting.

FIG. 11 is a view comparing the torsion coil spring 60 before and afterthe third step (twisting step). The torsion coil spring 60 beforetwisting is shown by a broken line, while the torsion coil spring 60after twisting is shown by a solid line.

If the other end 64 is twisted by the twisting angle β in the clockwisedirection about the axial center L1 in the state engaged with one end62, the outside diameter of the torsion coil spring 60 is compressedfrom D4 to D5. At this time, the position separated from the one end 62of the spiral part 66, in particular the position far from the bent part63 engaged with the base 30, is shifted to the base axial center L1side. The spiral axial center L3 deviates from the initial positionshown by the white circle to a position close to the bent part 63 shownby the black circle. The spiral axial center L3 after assembly of theautotensioner 20 is eccentric with respect to the base axial center L1in the top right direction in the drawing, i.e., the directionsubstantially opposite to the axial load center Y.

The outside diameter D5 of the spiral part 66 after twisting is set to avalue smaller than the diameter D2 of the ring-shaped chamber 100.Namely, the outer circumference of the spiral part 66 is set to a valuesmaller than the diameter D2 by the clearance for separating the outercircumference of the spiral part 66 to an extent so as not to interferewith the arm inner circumferential surface 246 b and the bottom innercircumferential surface 34 b.

Thus, the torsion coil spring 60 is housed in the base 30 in aneccentric and twisted state, so that the rocking arm 24 is pushed in thepushing direction Z substantially coinciding with the axial loaddirection Y, and the rocking arm 24 is made to tilt. Therefore, it ispossible to increase the force by which the rocking arm 24 pushes thebushing 26 when the belt is tensed, and possible to set the firstdamping force to an extremely large value and enhance the dampingeffect. The process of assembly of the torsion coil spring 60 is easy asdescribed above.

The eccentric position and amount of eccentricity of the spiral axialcenter L3 after assembly of the autotensioner 20 are determined by theangle α (see FIG. 8) formed by the two ends 62 and 64, the twistingangle β, and the positions of the engagement projections 322 and 324 andthe outer circumference 324 b of the base 30. These values and positionsand the diameter D2 of the ring-shaped chamber 100 housing the torsioncoil spring 60 and the number of turns and outside diameter D4 of thenatural length of the torsion coil spring 60 are not limited to theembodiment. It is of course possible to change the design to the valuesand positions bringing out most effectively the damping performance ofthe autotensioner 20.

The characteristics of an autotensioner 20 will be explained below withreference to graphs of FIGS. 12 a and 12 b. FIG. 12 a is a graph of theoutput characteristic of the autotensioner 20 without the bushing 26 andprovided with only the torsion coil spring 60. In this graph, therotational angle of the rocking arm 24 from a predetermined initialposition is the abscissa, and the output load of the autotensioner 20 isthe ordinate.

When the rocking arm 24 rotates from the initial position to therotational angle K1, i.e., rotates forward, since only theproportionally increasing twisting torque acts on the rocking arm 24,the forward operation load Ca output from the autotensioner 20 increasesproportionally along with the increase of the rotational angle. When therocking arm 24 rotated to the rotational angle K1 returns to the initialposition due to the twisting torque of the torsion coil spring 60, i.e.,rotates in reverse, since the twisting torque falls proportionally, thereverse operation load Cb output from the autotensioner 20 falls inproportion to the reduction in the rotational angle. The lines showingthe forward operation load Ca and the reverse operation load Cbsubstantially match, and the inclinations of the lines match with thetorsion spring constant of the torsion coil spring 60.

FIG. 12 b is a graph of the output characteristics of the autotensioner20 provided with both of the bushing 26 and torsion coil spring 60. Forreference, the output characteristic of a torsion coil spring 60 alone(forward operation load Ca and reverse operation load Cb) is shown by aone-dot chain line.

The forward operation load Ta when providing the bushing 26 is larger bythe load Pa (Pa=Ta−Ca) from the forward operation load Ca at the time ofthe torsion coil spring 60 alone. This load Pa corresponds to thefrictional resistance caused by the bushing 26, i.e., the first dampingforce. Further, the reverse operation load Tb when providing the bushing26 is smaller by the load Pb (Pb=Tb−Cb) from the reverse operation loadCb at the time of the torsion coil spring 60 alone. This load Pb is thefrictional resistance caused by the bushing 26, i.e., the second dampingforce.

As shown in FIG. 12 b, the second damping force Pb is substantiallyconstant from the initial position to the angle K1. The first dampingforce Pa becomes gradually larger as the rotational angle becomesgreater, and is always larger than the second damping force Pb. Thus, byproviding the bushing 26, it is possible to impart a damping force Pa orPb with a different magnitude in accordance with the direction ofrotation of the rocking arm 24. The ratio of magnitude of the firstdamping force Pa and the second damping force Pb is Pa:Pb=1.5 to 3.5:1.This ratio can be set to any ratio by changing the frictionalcoefficient of the bushing 26 and the outside diameter of the armtubular part 246.

As described above, in the autotensioner 20, a clearance is providedbetween the rocking arm 24 and stepped bolt 40 and the base 30 to allowrelative displacement of the rocking arm 24, and the torsion coil spring60 is made eccentric and pushes the rocking arm 24 in the pushingdirection Z substantially matching with the axial load direction Y. Dueto this, when the rocking arm 24 moves in the A direction (FIG. 1), thetorsion coil spring 60 is twisted, the rocking arm 24 displaces relativeto the axial load direction Y so that the rocking arm 24 is pushedstrongly against the bushing 26, and the clockwise rotation of therocking arm 24 is braked by the relatively large first damping force.Conversely, when the rocking arm 24 moves in the B direction, thetwisting of the torsion coil spring 60 is released, the rocking arm 24separates from the bushing 26, so that the second damping force becomessmaller and the rocking arm 24 can easily rotate counterclockwise.Namely, the damping performance of the autotensioner 20 can be improved,and the tracking ability becomes extremely good.

A second embodiment of the autotensioner will be described below withreference to FIG. 13 and FIG. 14. FIG. 13 is a plan view of anautotensioner, and shows only part of the bushing and rocking armattached to the base. FIG. 14 is a perspective view of the bushing. Theautotensioner of the second embodiment is provided with the sameconfiguration as the first embodiment except for the point of thedifferent shape of the bushing. The same parts are assigned the samereference numerals, and explanations thereof are omitted.

The bushing 426 of the second embodiment is a semitubular memberprovided extending over a range of 180 degrees around the base axialcenter L1. The center in the circumferential direction is on the axialload direction Y. Namely, the bushing 426 slides against the arm outercircumferential surface 246 a across a range of ±90 degrees from theaxial load direction Y.

The bushing 26 in the first embodiment is a tubular shape, and theportion most receiving the load is the pushing position 26 w in theaxial load direction Y (FIG. 4). This receives the load for exactly therange of about ±90 degrees from the axial load direction Y. Therefore,at the position at the opposite side, the bushing 26 is separated fromthe arm outer circumferential surface 246 a, and no frictional force isgenerated. Due to this, in the second embodiment, the semitubularportion extending over the range of ±90 degrees from the axial loaddirection Y required for the frictional sliding is used as the bushing426. Even with this shape, it is possible to obtain similar effects asthe first embodiment. Note that, while not shown, it is preferable toprovide a rotation stop for positioning the bushing 426 at the bushing426 or base 30. Thus, according to the second embodiment, similar to thefirst embodiment, it is possible to improve the damping performancewithout reducing the following ability and to further reduce thematerial used from the first embodiment.

A third embodiment of the autotensioner will be described below withreference to FIG. 15 and FIG. 16. FIG. 15 is a plan view of anautotensioner, and shows only part of the bushing and rocking armattached to the base. FIG. 16 is a perspective view of the bushingpartially cut away. The autotensioner of the third embodiment isprovided with the same configuration as the first embodiment other thanthe point of the different shape of the bushing. The same parts aregiven the same reference numerals, and explanations thereof are omitted.

The bushing 526 is provided with two projections 552 and 554 projectingout to the inside in the radial direction. The arm outer circumferentialsurface 246 a closely contacts the bushing 526 only at the projections552 and 554, and does not contact the other portions of the bushing 526.The projections 552 and 554 are provided across the entire axis of thebushing 526, and are 45 degrees apart from the axial load direction Y(pushing direction Z) in the circumferential direction. The forcepushing the bushing 526 when the belt 10 is wound concentrates in theaxial load direction Y. In the third embodiment, however, theprojections 552 and 554 receive the load, so that the load is dispersed.Further, since the projections 552 and 554 are at positions of 45degrees with respect to the axial load direction Y, the load appliedbecomes 1/{square root}2 of the load in the axial load direction Y.Therefore, according to the third embodiment, in the same way as thefirst embodiment, not only is it possible to improve the dampingperformance without reducing the following ability, it is also possibleto prevent early damage and early abrasion of the bushing 526, andpossible to improve the endurance of the autotensioner 20.

Note that the angle formed with respect to the axial load direction Y ofthe projections 552 and 554 is not limited to the above 45 degrees, andmay also be 30 degrees or 60 degrees. Further, the bushing 526 is atubular member, but may also be a semitubular member such as in thesecond embodiment.

A fourth embodiment of the autotensioner will be described below withreference to FIG. 17 and FIG. 18. FIG. 17 is a sectional view of anautotensioner, while FIG. 18 is a perspective view of a bushing. Theautotensioner of the fourth embodiment is provided with the sameconfiguration as the first embodiment other than the point of thedifferent shapes of the bushing and thrust bearing, and the point offurther provision of the damping mechanism. Similar parts are given thesame reference numerals plus 600, and explanations thereof are omitted.Note that the pulley and surrounding parts are shown by broken lines.

In the autotensioner 620 of the fourth embodiment, a ring-shaped chamber700 is formed by the base bottom 632, the arm disk 842, the bottom innercircumferential surface 634 b of the base 630, the arm innercircumferential surface 846 b, the base shaft hole part 638, and therocking shaft 844. The ring-shaped chamber 700 houses not only thetorsion coil spring 660, but also the damping mechanism.

In the first embodiment, the only member imparting frictional resistanceto the rocking arm 24 is the bushing 26, and sometimes a sufficientdamping force may not be obtained with just the bushing 26. The fourthembodiment is further provided with a separate damping mechanismimparting a frictional force to the rocking arm 624 to meet with thisdemand, so that a high damping force is generated.

The damping mechanism is provided with a second tubular part 702integral with the base 30, a tubular damping member 704 attached to therocking arm 624, and a ring spring 706 pushing the damping member 704from the inside to the second tubular part 702. Since theseconfigurations are the same as the damping mechanism shown in JapanesePatent No. 2981433, a detailed explanation is omitted. The dampingmember 704 rotates integrally with the rocking arm, and frictionallyslides with the second tubular part 702. The frictional force generatedat this time is proportional to the biasing force of the ring spring706. Not only the frictional force generated at the bushing 626, butalso the frictional force generated at the damping member 704 acts onthe rocking arm 624. Thus, it is possible to apply stronger braking tothe rocking arm 624.

A clearance allowing relative displacement of the rocking arm 624 isprovided between the mounting hole 710 of the damping member 704 and themounting pin 712 of the rocking arm 624 engaged with the mounting hole710. Due to this, when the rocking arm 624 displaces, strain and damageare prevented from occurring at the damping member 704. Further,regardless of the displacement of the rocking arm 624, it is possible togenerate a stable frictional force.

The thrust bearing 650 is not a ring-shaped member, but a tubular memberprovided with a flange. The thrust bearing 650 is fit snugly into thebase shaft hole part 638, and faces the cylindrical part 646 of thestepped bolt 640 passed through the inside with a clearance. Regardingthe base axial center L1 direction, the thrust bearing 650 is heldwithout clearance by the bolt head 644 and the base bottom 632. Due tothis, the relative displacement of the rocking arm 624 is allowed, andabrasion due to mutual interference between the base 30 and the steppedbolt 40, which are formed from the metal material, can be prevented.Note that the tubular part of the thrust bearing 650 may be formed in ataper becoming smaller in diameter toward the rocking arm 624 side.

The bushing 626 has a tapered cross-section where the diameter becomesgradually smaller toward the base bottom 632. A flange 862 extendingtoward the inside of the radial direction is formed integrally with oneend of the tubular part across about 180 degrees. The flange 862 issandwiched between the base 30 and the front end of the rocking armtubular part 846, and has the function of not only preventing thebushing 626 from detaching from the base 30, but also stopping rotationin the circumferential direction. While not shown, the ring-shaped seat634 c of the base 630 is formed in a step shape to engage with theflange 862 at the circumferential direction. The flange 862 ispreferably provided across a range of +90 degrees from the axial loaddirection where the tilt of the rocking arm 624 is the greatest toprevent extreme tilting of the rocking arm 624.

The rocking arm bottom 842 is formed across the entire circumferencewith a flange 720 covering the bushing 626. The flange 720 covers thetop end of the bushing 626, and extends at a slant downward in thedrawing up to the opening of the base tubular part 634. The top end ofthe bushing 626 is protected from the outside by the flange 720, so thatthe entry of dust, water, salt water, etc. to the surface of the bushing626 is prevented. Further, the damping member 704 provided inside thering-shaped chamber 700 is also protected simultaneously from dust,water, salt water, etc.

As described above, even in the autotensioner 620 of the fourthembodiment, in the same way as the first embodiment, the damping forceis changed by displacing the rocking arm, so that the dampingperformance can be improved without reducing the following ability.Further, by providing the damping member 704, it is possible to set ahigh damping force. Further, by providing the thrust bearing 650 betweenthe shaft hole part 638 and the bolt 640 and providing the flange 720covering the end of the bushing 626, it is possible to prevent earlydamage and early abrasion of the base 30 or bushing 62, and possible toimprove the endurance of the autotensioner 620.

The autotensioner 20 was tested for endurance, and the change in theratio of the damping force and first and second damping forces alongwith the elapse of time was investigated. FIG. 19 is a view of thelayout showing the state of an endurance test, while FIG. 20 is a graphof the results of measurement of an autotensioner in the initial state.

In the endurance test, the output load when removing the bushing 26 fromthe autotensioner and providing only the torsion coil spring 60, and theoutput load when providing the bushing 26 of the first embodiment (seeFIG. 2), were measured in the initial state immediately after assembly.Further, a separate autotensioner provided with the same structure asthe autotensioner used for measurement of the initial state was used forthe belt transmission mechanism shown in FIG. 1 for 180 hours, and inthe fatigue state after the elapse of 180 hours, the output load whenproviding only the torsion coil spring 60 and the output load whenproviding the bushing 26 were measured.

In the measurement of the output load, a V-block 90 was made to abutagainst the side surface of the pulley 22 to push the pulley 22 in onedirection to make the rocking arm 24 rotate forward, then the V-block 90was returned to make the rocking arm 24 rotate in reverse. The loadreceived by the V-block 90 from the pulley 22, i.e., the output load ofthe autotensioner 20, was measured by a detector 92 attached to theV-block 90.

Table 1 shows results of measurement of the forward operation load andreverse operation load of the autotensioner in the initial state whenthe rocking arm 24 is in the first position shown by the solid line inFIG. 19 (rotational angle of 28 degrees) and the second position shownby the broken line in FIG. 19 (rotational angle of 40 degrees), thefirst and second damping forces Pa and Pb at the different positions,and the ratio of the two (Pa/Pb). TABLE 1 Damping Autotens. Torsion coilDamping ratio overall spring alone force Pa/Pb First Forward Ta1 = 894Ca1 = 605 Pa1 = 289 Pa1/Pb1 = pos. Reverse Tb1 = 473 Cb1 = 605 Pb1 = 1322.19 Second Forward Ta2 = 949 Ca2 = 66l Pa2 = 288 Pa2/Pb2 = pos. ReverseTb2 = 521 Cb2 = 661 Pb2 = 140 2.06

Table 2 shows results of measurement of the forward operation load andreverse operation load of the autotensioner in the fatigue state whenthe rocking arm 24 is in the first position and the second position, thefirst and second damping forces Pa and Pb at the different positions,and the ratio of the two (Pa/Pb). The dimensions, shape, and material,that is, frictional coefficient, of the bushing 26 used here and theoutside diameter of the rocking arm 24 (arm tubular part 246) are thesame as those of the autotensioner measured in the initial state. TABLE2 Damping Autotens. Torsion coil Damping ratio overall spring aloneforce Pa/Pb First Forward Ta1 = 793 Ca1 = 571 Pa1 = 222 Pa1/Pb1 = pos.Reverse Tb1 = 447 Cb1 = 571 Pb1 = 124 1.79 Second Forward Ta2 = 853 Ca2= 620 Pa2 = 233 Pa2/Pb2 = pos. Reverse Tb2 = 493 Cb2 = 620 Pb2 = 1271.83

The results of the experiments shown in the above two tables areexamples. According to the above results of experiments and the resultsof other experiments on the autotensioner changing the frictionalcoefficient of the bushing 26, the outside diameter of the arm 24, theendurance, etc., it is learned that the first damping force Pa is alwayslarger than the second damping force Pb, the ratio Pa/Pb is in the rangeof 1.5 to 3.5, and a proportional relationship always stands both at thefirst position and second position and even after fatigue.

Below, an example of the bushing 26 will be described. As an example, abushing is formed from a material mainly comprised of a polyphenylenesulfide resin (PPS), while as a comparative example, a bushing wasformed by a material mainly comprised of a partial aromatic polyamideresin (PPA). FIG. 21 shows the results of a water spraying testconducted using the bushing of the example and the bushing of thecomparative example.

The water spraying test was performed by attaching the bushing of theexample of the invention and the bushing of the comparative example tothe autotensioner 20 shown in FIG. 2. In the test, the base 30 was fixedin place, the rocking arm 24 was made to rock by a rocking amplitude of±1.5 mm, a frequency of 25 Hz, and a room temperature environment, andwater was sprayed toward the clearances of the rocking arm 24 and thebase 3 b. Further, the output load of the autotensioner 20 was measuredevery 10 minutes. Note that the “forward operation load” shown in thetable means the output load when the belt 10 is pushed against and therocking arm 24 rotates in the counterclockwise direction of FIG. 1,while the “reverse operation load” means the output load when therocking arm 24 rotates in a direction tensing the belt 10 (clockwisedirection of FIG. 1).

As is clear from the graph of FIG. 21, in the case of the bushing of thecomparative example, the reverse operation load does not change much atall, but the value of the forward operation load rises along with theelapsed time. This shows that the frictional force arising due to thefrictional sliding of the bushing of the comparative example and therocking arm 24, in particular the frictional force arising when therocking arm 24 rotates counterclockwise, rises due to spraying by water.Therefore, if using a bushing 26 formed by a material mainly comprisedof a PPA resin, the frictional force becomes too high when water issprayed, a stick-slip phenomenon arises, and there is the danger of theeasy occurrence of abnormal noise.

Conversely, in the case of the bushing of the example, in the same wayas the comparative example, the reverse operation lcad is substantiallyconstant, and the forward operation load is substantially constantregardless of the elapsed time though it is slightly smaller in valueafter spraying with water than in the initial state (elapsed time of 0).Namely, from the results of the test, it was learned that if using abushing formed by a material mainly comprised of a polyphenylene sulfideresin, the frictional force does not change even when water is sprayed,and the response of the belt to tension does not deteriorate.

Although the embodiments of the present invention have been describedherein with reference to the accompanying drawings, obviously manymodifications and changes may be made by those skilled in this artwithout departing the scope of the invention.

The present disclosure relates to subject matter contained in JapanesePatent Application No. 2001-114769 (filed on Apr. 13, 2001), No.2001-115003 (filed on Apr. 13, 2001), and No. 2001-129141 (filed on Apr.26, 2001) which are expressly incorporated herein, by reference, intheir entirety.

1. An autotensioner comprising: a base that has a bottomed tubularshape; a rocking arm that has a tubular part rotatably supported at theinside of said base; a pulley that is attached to one end of saidrocking arm, and abuts against a belt; a torsion coil spring that ishoused in said base, and biases rotation of said rocking arm in adirection tensioning said belt with respect to said base; said torsioncoil spring being attached eccentrically to the axial center of saidbase, and said rocking arm being supported to be able to be displacedrelative to said base, whereby the first damping force acting on saidrocking arm when said belt is tensioned becomes relatively larger thanthe second damping force acting on said rocking arm when said belt isslack; a friction member that is interposed between an outercircumferential surface of said tubular part and an innercircumferential surface of said base, and provided across a range of atleast 180 degree around the axial center of said base, a part of saidtubular part being biased to be pushed against said friction member bysaid torsion coil spring; and a damping member separate from saidfriction member, said damping member engaging with said rocking armmovably in the radial direction and frictionally sliding with said base.2. A thin autotensioner comprising: a base that has a cup having aninside diameter; a rocking arm that is rotatably supported by said base;and a torsion coil spring that biases said arm in a predetermineddirection, said torsion coil spring having an outside diameter largerthan said inside diameter, said torsion coil spring being twisted in adirection in which said outside diameter is compressed so as to behoused inside said cup.
 3. A thin tensioner according to claim 2,wherein said torsion coil spring is engaged at one end with a firstengagement part provided inside said base, and engaged at the other endwith a second engagement part provided inside said rocking arm, andrelative positions of said first engagement part and said secondengagement part differing when said torsion coil spring engages with thefirst engagement part and the second engagement part and when saidrocking arm is attached to said base.
 4. A thin tensioner according toclaim 2, wherein an axial length of said torsion coil spring is shorterthan said outside diameter.
 5. A thin tensioner according to claim 2,further comprising at least one friction member that is interposedbetween said cup and said rocking arm, and gives a frictional resistanceto the rocking of said rocking arm.
 6. A thin tensioner according toclaim 5, wherein said friction member is composed of a tubular part, anda flange projecting from a bottom of said tubular part to an insidedirection of said cup and rocking arm, said friction member exhibitingan L-shape in cross-section.
 7. A method of assembly of a thin tensionercomprising: a first step of twisting a torsion spring coil having anoutside diameter larger than an inside diameter of a cup to make theoutside diameter smaller than said inside diameter; and a second step ofinterposing said twisted torsion coil spring between said cup androcking arm.
 8. A method of assembly of a thin tensioner comprising: afirst step of engaging one end of a torsion coil spring having anoutside diameter larger than an inside diameter of a cup, with said cup;a second step of engaging another end of said torsion coil spring with arocking arm; a third step of rotating said rocking arm to twist saidtorsion coil spring and make the outside diameter smaller than saidinside diameter; a fourth step of bringing said rocking arm intoproximity with said cup to compress said torsion coil spring and houseit in said cup; and a fifth step of rotatably fastening said rocking armto said cup.
 9. An autotensioner comprising: a base that has a firsttubular part having a bottomed tubular shape; a rocking arm that has asecond tubular part, which is attached rotatably to an open side of saidbase and is separated by a certain distance from said first tubular partin the radial direction; and a friction member that is provided betweensaid first tubular part and said second tubular part, and brakes saidrocking arm; said friction member being partially exposed, and beingformed from a material mainly comprised of a polyphenylene sulfideresin.
 10. An autotensioner according to claim 9, further comprising atorsion coil spring that is provided at an inside of said second tubularpart, to bias said rocking arm in a certain rotational direction andpush said second tubular part and said friction member toward said firsttubular part.
 11. An autotensioner according to claim 10, furthercomprising a rocking shaft member that supports said rocking armrotatably with respect to said base, and passes through the bottom partof said base while forming a clearance with respect to said base.
 12. Anautotensioner according to claim 9, wherein said friction member ispartially cut away in the circumferential direction.
 13. Anautotensioner according to claim 9, wherein said friction member has aplurality of grooves on a surface of said friction member, which surfacefrictionally slides with said rocking arm, said grooves extending acrossthe entire axis of said friction member.
 14. An autotensioner accordingto claim 9, wherein an axial length of said first tubular part and theaxial length of said second tubular part are substantially equal, andsaid friction member is in close contact with said first tubular partand said second tubular part across the entire axis of said frictionmember.
 15. A friction member in an autotensioner rotatably attaching arocking arm to a base, characterized by being provided between saidrocking arm and said base, and being formed by a material mainlycomprised of a polyphenylene sulfide resin.